The invention concerns a calendering method, in which the material web to be calendered, in particular a paper or board web, is passed through the calender, in which calender the calendering nip is formed between a heatable hard roll and an endless, flexible and substantially non-compressible calendering belt passed over said roll, in which connection, in the method, the heatable hard roll is heated in order to plasticize the surface layer of the material web to be calendered placed at the side of the heatable roll, and in which connection the material web to be calendered is brought into a preliminary contact with the heatable roll before the calendering proper of the web, i.e. before the stage in which the material web is pressed between the heatable hard roll and the calendering belt.
Also, the invention concerns a calender that makes use of the calendering method, comprising a calendering nip which is formed between a heatable hard roll and an endless, flexible and substantially non-compressible calendering belt passed over said roll, through which calendering nip the material web to be calendered, in particular a paper or board web, has been fitted to pass, in which connection the heatable hard roll has been arranged to be heated in order to plasticize the surface layer of the material web to be calendered that is placed at the side of the heatable roll, and in which connection the material web to be calendered has been passed into a preliminary contact with the heatable roll before the calendering proper of the web, i.e. before the stage in which the material web has been arranged to be pressed between the heatable hard roll and the calendering belt.
When it is desirable to improve the standard of calendering, with the present solutions, in actual fact, the only possibility is to increase the number of calendering nips. This results in a more complicated construction of the calender and in more difficult control and tail threading of the paper web. Especially in the case of on-line machines, it must be possible to solve the contradictions arising from high running speed and from threading at full speed, Attempts have been made to solve these problems by means of various belt and shoe calenders, by whose means the calendering nip is extended and, thus, the operation of the nip is made more efficient. For example, in belt calenders, which in themselves are relatively recent constructions, the paper to be calendered is passed by means of an endless belt into a preliminary contact with a hot calender roll, in which case it is possible to create a steep temperature gradient, which is favourable from the point of view of calendering. By means of the belt, the effective length of the nip is increased, owing to the preliminary contact and because, as the belt material, it is possible to use considerably softer polymers than in roll coatings without problems arising from deformations related to heat. With a nip longer than in a supercalender or soft calender, the press impulse applied to the paper can be increased so that the pressure peak does not become excessively high and that the bulk does not start decreasing.
One belt calender solution has been described in the prior art, for example, in the Finnish Patent No. 95,061. A calender embodiment in accordance with said publication is illustrated schematically in FIG. 1A in the drawing, which figure represents the prior art. Thus, FIG. 1A is a schematic illustration of a prior-art calender, in which the calendering nip N is formed between a heatable hard roll 1 and a calendering belt, in particular a metal belt 5, supported by a roll 2 with resilient coating. The metal belt 5 is an endless belt, and its material can be, for example, steel. The belt is passed over a nip roll 2 provided with a resilient coating 3 and over a reversing roll 4. As was already stated above, in this prior-art calender, the calendering nip N is, thus, formed between a heatable hard roll 1 and said metal belt 5, which is supported by a calender roll 2 provided with a resilient coating. Such a solution is, in fact, quite extensively similar to a nip in a soft calender, in which, however, by means of the metal belt 5, both faces of the paper W can be subjected to a substantially equal treatment and, thus, the glazing can be made to take place at both sides of the paper W at the same time.
Further, FIG. 1B illustrates a further development of the prior-art calender as shown in FIG. 1A. In the illustration in FIG. 1B, the calender has been extended to be a calender with two nips, so that the calender comprises two heatable hard-faced calender rolls 1A and 1B, two calender rolls 2A,2B provided with resilient roll coatings 3A, 3B, and an endless metal belt 5. The endless metal belt 5 is passed over said rolls 2A,2B with resilient coatings, and said rolls with resilient coatings form calendering nips N2,N2 with the heatable hard-faced rolls 1A,1B. More correctly, the calendering nips are formed, in each particular case, between the heatable hard-faced roll 1A,1B and the metal belt 5, which metal belt 5 is loaded by means of a corresponding roll 2A,2B with a resilient coating against the heatable hard-faced roll 1A,1B. As is shown in FIG. 1B, the paper web W is passed through the first nip N1, after which the web is spread and reversed by means of the take-out leading rolls 4A,4B and guided into the second nip N2. The construction and the operation of the rolls 2A,2B with resilient coatings are similar to the illustration in FIG. 1A. The prior-art calenders as shown in FIGS. 1A and 1B are suitable for use with paper grades that do not require a long nip time to be glazed. Such paper grades are, for example, coated grades in which the glass transition temperature of the coating paste is low and in which, therefore, the glazing is rapid.
In view of on-line operation, a belt calender provides a significant advantage, among other things, in respect of the clear and linear running of the paper web, which again permits tail threading taking place at a high speed. In a belt calender supported by means of a glide shoe, the nip is formed between an endless belt and a steel roll. Owing to the glide shoe, the press zone becomes wider than in the belt calender described above. In a nip in a shoe calender, owing to the wider nip, the maximal pressure remains lower than in other present-day calenders, for which reason it is best suitable for paper grades in which retaining of the bulk has a high importance. As regards its construction, such a shoe calender is quite extensively similar to extended-nip presses, which have already been in use for a rather long time.
In respect of the prior art, as an example that represents shoe calendering, reference can be made, e.g., to the Published German Patent Application No. 43 44 165, wherein a smoothing method is described in which the fibrous web is passed in between two heatable faces which have been fitted at both sides of the web and which can be pressed against the web. The compression pressure can be regulated both in the running direction of the web and in the cross direction of the web in a way that has been chosen in advance. The prior art also includes the U.S. Pat. No. 5,163,364, which concerns a similar equipment provided with a glide shoe.
In respect of belt-supported calender concepts, reference is made additionally to the U.S. Pat. No. 4,596,633, in which a web finishing process is described, wherein the surface portions of the web to be finished are first moistened to a high degree of moisture (dry solids content 50% . . . 70%), and the web is then passed, on support of a belt, into a long finishing zone of low pressure, which zone comprises more than one roll nips. In said method, as the belt, prior-art paper machine fabrics are employed, such as felts, wires or polymer belts, while the surface treatment proper is applied exclusively to the side of the web placed facing the backup roll.
In surface treatment devices provided with a glide shoe, it can be considered that the, at least partly dragging, contact between the belt and the glide shoe is a problem, which contact applies quite a high strain both to the glide shoe and also to the belt. When a technology commonly employed in initial drying of a fibrous web has been applied, it has been realized that the quality of a belt that is well suitable for initial drying and that operates well in initial drying is inadequate in conditions of finishing of the web surface, in particular in respect of its resistance to the higher strains applied to the belt. Also, of course, the high local strains applied to the belt in web break situations are an almost equally important problem as in polymer-coated rolls, even though a similar polymer present in belt form tolerates considerably higher strains than a coating attached to a roll face rigidly does. In shoe calenders with solutions provided with a glide shoe and a belt, it is a further problem that the ends of the belt must always be closed, or spreading into the environment of the fluid and/or evaporation product employed in order to reduce the glide friction must be prevented in some other way.
A long belt circulation and a roll nip involve a similar basic problem. The quality of the belt face and a homogeneous inner structure are an unconditional requirement in order that a uniform quality of web surface could be achieved, and, moreover, keeping the long belt loops, which are made of a more or less elastic material and which often comprise more than on belt alignment roll, in their position in the cross direction of the web requires the construction of a regulation system of remarkable complexity in connection with the finishing device.
The present invention concerns a calendering method and in particular a calendering method that makes use of a metal belt calender, in which, by means of an endless metal belt, a calender with a very long nip is provided so that attempts are made to create all stages of the calendering process in one and the same calendering nip without unnecessary additional operations. The basic idea of the invention is to prevent contact between the web to be treated and soft roll coatings while, however, taking advantage of the effect of extending the calendering zone of said soft roll coatings. Thus, the object of the invention is to provide a calendering method that has been improved substantially, as compared with the prior art, and a calender that operates in accordance with this improved method, by means of which calender the calendering process can be made readily controllable and by means of which method the construction of the calender that carries out the method can be made relatively simple. In view of achieving the objectives of the invention, the method in accordance with the invention is mainly characterized in that, after the stage of preliminary contact, the press treatment proper is applied to the material web to be calendered, i.e. the web is calendered in at least two stages so that a deformation is produced in the material web so that it is first pressed in the press stage between the heatable hard roll and the calendering belt by means of a first roll provided with a resilient coating, the deformation that was produced in the press stage is allowed to be reversed partially under control in a reversing stage by still keeping the material web supported between the heatable hard roll and the calendering belt, and a new deformation is produced in the material web, which was already once pressed and partly reversed, by pressing the web again in a finishing press stage between the heatable hard roll and the calendering belt by means of a second roll provided with a resilient coating.
On the other hand, the calender in accordance with the invention is mainly characterized in that, in view of carrying out the press treatment proper, i.e. the calendering of the web, after the stage of preliminary contact, in the running direction of the calendering belt, at least two calender rolls provided with resilient coatings have been fitted one after the other at a distance from one another to press the calendering belt against the heatable hard roll so that the nip is composed of the press zone proper placed between the first calender roll with a resilient face and the heatable roll, of a zone of reversing of the deformation, following after said press zone, in which zone the material web to be calendered is supported between the calendering belt and the hot roll, and of a finishing press zone following after the reversing zone and placed between the second calender roll with a resilient face and the heatable roll.
By means of the invention, as compared with the prior-art calendering methods and calenders, a number of significant advantages are achieved, of which, for example, the following can be stated in this connection.
When, in the present invention, a flexible, thin and substantially non-compressible belt is used in a novel way together with rolls provided with compressible or non-compressible coatings, a very wide range of regulation of pressure is obtained together with a simultaneous range of high running speeds. Further, when a substantially non-compressible belt, whose material can be metal or, for example, a hard polymer, such as a fibre-reinforced resin, and a roll that is provided with a resilient coating and that supports said belt at the nip are employed, a resilient finishing zone is obtained which has a face of very high quality and which is adapted against the web face very well in compliance with the loading. Further, by means of the solution in accordance with the invention, a finishing device is provided in which the overall length of the web treatment zone is very long and, if necessary, includes a number of zones with different pressure ranges. An essential feature of a calender in accordance with the present invention is the hardness of the calendering belt that is used, as compared with the roll coating. This provides the highly significant advantage that tail threading is free of problems and easy, for the leader end of the web can be passed through the calender as of full width. The further advantages and characteristic features of the invention will come out from the following detailed description of the invention.